Apparatus for reacting a particulate solid and a liquid

ABSTRACT

An apparatus useful for the preparation of phosphoric acid from phosphate rock and a strong acid, comprises a closed vessel, a draft tube, means connected to the inner walls of said vessel amd mounting said draft tube in a vertical position within said vessel, an agitator positioned within said draft tube, a shaft for said agitator mounted axially of said vessel and extending into said draft tube, an inlet conduit to said vessel for introducing a feed slurry (e.g. phosphate rock and strong acid) into said vessel, said inlet conduit preferably having a lower end portion terminating within said draft tube and wherein, said draft tube has an outwardly flared lower skirt portion terminating in the bottom portion of said vessel, and preferably including a vent pipe connected to said inlet conduit to reduce foaming generated by the reaction in said vessel and to prevent blockage of said inlet conduit, and including means for applying air, under pressure, to the bottom of said reactor and/or under the bottom of said lower skirt portion, to prevent settling of undissolved solids.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 909,896, filed May 26,1978, now abandoned.

This application is related to applications Ser. No. 703,139 now U.S.Pat. No. 4,140,748 issued on Feb. 20, 1979, 703,138 now abandoned andSer. No. 703,208 now U.S. Pat. No. 4,132,760 issued on Jan. 12, 1979,all filed July 7, 1976, Ser. No. 676,559 filed Apr. 13, 1976 nowabandoned, Ser. No. 810,848 filed June 27, 1977 now U.S. Pat. No.4,136,199 issued on Jan. 23, 1979, Ser. No. 840,791 filed Oct. 11, 1977and to the application Ser. No. 865,557, now U.S. Pat. No 4,196,172issued on Apr. 1, 1980 of Ore', Moore, and Ellis titled "HemihydrateType Phosphoric Acid Process" filed Dec. 29, 1977 (the entiredisclosures of all the above-cited applications are incorporated hereinby this reference.)

BACKGROUND

The present invention especially relates to the processes described inapplications Ser. Nos. 676,559; 703,138; 703,139; 703,208, 865,556 nowabandoned; 865,557 now U.S. Pat. No. 4,196,172 issued Apr. 1, 1980;866,963 now U.S. Pat. No. 4,220,630 issued Sept. 2, 1980 and those ofOre' et al Ser. No. 910,163, now U.S. Pat. No. 4,260,584 filed of evendate since it permits reaction or dissolution of phosphate rock withdecreased settling of solids in all of these processes or systems.

In these applications, phosphate rock and sulfuric acid are reactedunder conditions which result in the formation of solid calcium sulfate(usually, hemihydrate or gypsum) and phosphoric acid. A two vesselreaction system is preferably used in which the reaction slurryundergoes intra- and inter-vessel circulation. Preferably, theinter-vessel circulation is through a draft tube. This results inexcellent dispersion of reactants and minimization of temperature andconcentration gradients throughout the slurry. In the hemihydrateprocess, the solution portion of the slurry in the first vessel (the"dissolver") is preferably maintained at a negative sulfateconcentration (i.e. excess dissolved Ca⁺²) and the solution in thesecond vessel (the "crystallizer") is preferably maintained at apositive sulfate ion concentration. Also preferred is that the secondvessel be maintained at a reduced pressure (e.g. to provide evaporativecooling). Better filtration rates can thus be obtained due to thefavorable shape, dominant size and size distribution (especially, lowfines content) of the calcium sulfate crystals. Most preferred is that acrystal modifier (e.g. a sulfonic acid, a sulfonic acid salt, tall oil,fatty acids or esterified tall oil, fatty acids) be present in thecrystallizer. A two vessel hemihydrate process is described in U.S. Pat.No. 3,418,077 of Robinson, issued Dec. 24, 1968. Also relevant is thearticle by J. G. Getsinger, "IV Hemihydrate by the Foam Process" inPhosphoric Acid, Part 1, edited by A. V. Slack, at pages 369-382, MarcelDekker, Inc., New York (1968), which shows a one vessel process.

U.S. Pat. No. 3,522,003 to Lopker shows CO₂ removal by venting but froma separate pipe from the feed inlet (see also 3,522,004).

The present invention is directed to the manufacture of phosphoric acidby the wet process wherein phosphate rock is dissolved or reacted inphosphoric acid to produce a solution or slurry of monocalciumphosphate. The hemihydrate, or as it is sometimes called, thesemihydrate, process is preferably employed to produce wet phosphoricacid from phosphate rock and sulfuric acid. Phosphate rock andphosphoric acid (which can contain H₂ SO₄ and is preferably pre-mixed ina separate slurry tank) are added to a first reaction vessel or set ofreactors, in parallel or series, (the "dissolver") which contains afirst slurry comprising calcium sulfate hemihydrate, monocalciumphosphate, and phosphoric acid. The "soluble" or "excess" sulfatecontent (i.e., the excess or deficiency of sulfate ions over calciumions) of the first slurry in the first reaction vessel can be maintained(e.g. by addition of sulfuric acid) at a concentration of about +0.7% toabout -4% or even -8% (more preferred 0.0 to -6%), as determined forexample, by the well-known gravimetric analysis. It is more preferred tobe at a negative sulfate (e.g. excess Ca⁺²). The sulfuric acid in thefirst (dissolver) vessel is usually contained in "recycle" phosphoricacid from a filtration step and/or sulfuric acid contained in a sidestream or recycle slurry from the second reaction vessel. If twodissolver vessels are in series, it is preferred that slurry from thesecond vessel (which has an excess of Ca⁺², therefore, no free sulfuricacid) is used as the dissolution medium in the first dissolver vessel(e.g., the first vessel would be at about -6% SO₄ and the second vesselat about -4%, caused by addition of sulfuric acid only to the secondvessel). This would greatly reduce the "lattice bound" P₂ O₅ loss.

Sulfuric acid is added to the second reaction vessel which contains asecond slurry comprising calcium sulfate hemihydrate, monocalciumphosphate, sulfuric acid and phosphoric acid. The sulfuric acid reactswith the monocalcium phosphate and any residual, undissolved phosphaterock, producing calcium sulfate hemihydrate and phosphoric acid. Thesoluble or excess sulfate concentration of the second slurry ismaintained at a positive value (about +0.7% to about +4.5%).

Sulfuric acid is added in amounts such that the sulfate content of theadded acid and the sulfate content of the added rock is equivalent toabout 90% to about 100% (more preferred 93-99.5%) of the stoichiometricamount of sulfate required to react with calcium added in the phosphaterock to form calcium sulfate hemihydrate.

As is well-known in the art, sulfate and/or sulfuric acid can beintroduced as such or as a part of a phosphoric acid "recycle" (as fromthe filtrate from filtering to separate the hemihydrate).

In order to maintain the desired soluble sulfate concentration in thefirst reaction vessel and in the second reaction vessel, circulationbetween the two reaction vessels is initiated. A first portion of thefirst slurry from the first reaction vessel is circulated through afirst conduit into the second reaction vessel, and a first portion ofthe second slurry from the second reaction vessel is circulated througha second conduit into the first reaction vessel. This circulation iscontinuous. In order to better disperse the added phosphate rock and theadded sulfuric acid within the slurry of the first and the secondreaction vessels respectively and to better disperse the incoming slurrywith the slurry present in the given reaction vessel, a second portionof the first slurry and a second portion of the second slurry iscirculated within the first and second reaction vessels, respectively,preferably each through its own draft tube preferably at a rate equal toat least 50% of the volume of the slurry in a given reaction vessel perminute. This inter- and intra-vessel circulation disperses the reactantswithin the slurry in the respective reaction vessels. A third portion ofthe second slurry is removed from the reaction system so as to separatethe liquid and solid components from the slurry.

Although such patents as U.S. Pat. Nos. 3,939,248 to Caldwell and2,968,544 to Zietz show the uses of draft tubes in phosphoric acidprocesses, these patents are not concerned with the problems of settlingof unreacted or undissolved solids.

Although the present process has been described as involving tworeaction vessels, it should be understood that additional vessels(including reaction vessels) can be useful in the process and, forexample, the dissolver can comprise two or more vessels in series or inparallel (e.g., see the applications of Ore et al filed of even dateherewith).

Especially preferred is the use of an additional vessel as a slurrytank, into which phosphoric acid (which can be a filtrate, usuallycontaining in the range of 0.5-3.5% sulfuric acid) and phosphate rockare contacted to form a slurry which is transported (as by an overflowpipe) to the dissolver vessel. In such a slurry tank, as in thedissolver, it is frequently useful to use venting and/or to add adefoaming agent. A preferred defoaming agent comprises tall oil, talloil fatty acids, or lower alkyl esters of tall oil fatty acids, ormixtures of such tall oil acids and esters, because such tall oiladditives can also function (alone or with added sulfonics) as crystalmodifiers. The preferred esters are the methyl and ethyl (as exemplifiedby the esters found in the commercial product marketed under thetradename AZ-10-A).

SUMMARY

An apparatus useful for the preparation of phosphoric acid fromphosphate rock and a strong acid, comprises a closed vessel, a drafttube, means connected to the inner walls of said vessel and mountingsaid draft tube in a vertical position within said vessel, an agitatorpositioned within said draft tube, a shaft for said agitator mountedaxially of said vessel and extending into said draft tube, an inletconduit to said vessel for introducing a feed slurry (e.g. phosphaterock and strong acid) into said vessel, said inlet conduit preferablyhaving a lower end portion terminating within said draft tube andwherein, said draft tube has an outwardly flared lower skirt portionterminating in the bottom portion of said vessel, and preferablyincluding a vent pipe connected to said inlet conduit to reduce foaminggenerated by the reaction in said vessel and to prevent blockage of saidinlet conduit, and inducing means for applying air, under pressure, tothe bottom of said reactor and/or under the bottom of said lower skirtportion, to prevent settling of undissolved solids.

The invention includes apparatus useful for the preparation ofphosphoric acid from phosphate rock and sulfuric acid, including incombination a first reaction vessel containing a first slurry (e.g.,comprising calcium sulfate hemihydrate, monocalcium phosphate andphosphoric acid), a second reaction vessel containing a second slurry(e.g. comprising phosphoric acid and calcium sulfate hemihydrate),circulation means including a draft tube disposed centrally in each ofsaid vessels and an agitator positioned axially in each of said vesselsand extending into said draft tube, means for conducting said firstslurry from said first reaction vessel to said second reaction vessel, asecond conduit interconnecting said vessels for conducting said secondslurry from said second reaction vesssel to said first reaction vessel,an inlet pipe for introducing phosphate rock and phosphoric acid to saidfirst reaction vessel, and wherein, preferably, a vent is connected tosaid inlet pipe to permit escape of gases and, thus, increasing the rateof flow of said phosphate rock and phosphoric acid to said firstreaction vessel, but especially where means are provided for introducingair under pressure to the bottom portion of said first reaction vesseland/or under the lower portion of the draft tube skirt.

The invention also includes apparatus useful for the preparation ofphosphoric acid from phosphate rock and sulfuric acid, including incombination:

a. A first reaction vessel containing a first slurry (e.g. comprisingcalcium sulfate hemihydrate, monocalcium phosphate and phosphoric acid).

b. A second reaction vessel containing a second slurry (e.g., comprisingcalcium sulfate hemihydrate, monocalcium, phosphate, sulfuric acid andphosphoric acid).

c. Means in each of said vessels for maintaining a continuouscirculation of the slurry there in from the bottom to the top of eachvessel and from the top to the bottom of said vessel.

d. A first conduit interconnection said first and second reactionvessels for conducting said first slurry from said first reaction vesselto said second reaction vessel.

e. A second conduit interconnecting said vessels for conducting saidsecond slurry from said second reaction vessel to said first reactionvessel,

f. Means for applying a vacuum to said second reaction vessel to effecttemperature control in said second reaction vessel and to form a vacuumseal between said first and second reaction vessels,

g. Means for introducing a reactive or dissolving liquid and aparticulate solid (e.g., phosphate rock and phosphoric acid) to saidfirst reaction vessel, and, preferably, including venting means toremove gases from said introducing means,

h. Means for introducing a reagent (e.g., sulfuric acid) to said secondreaction vessel,

i. Means for withdrawing a slurry (e.g., containing phosphoric acid andcalcium sulfate hemihydrate) from said second reaction vessel; and

j. means for introducing a fluid under pressure (e.g., air) to thebottom portion of said first reaction vessel and/or under the lowerskirt portion of the draft tube therein.

Preferably the system also includes a third reslurry vessel forreslurrying phosphate rock and recycle phosphoric acid, and a thirdconduit interconnecting said third vessel with said means forintroducing phosphate rock and phosphoric acid into said first reactionvessel.

Also preferred is that said means for maintaining a continuouscirculation of the slurry in each of said first and second reactionvessels includes a draft tube disposed centrally in each of said vesselsand an agitator positioned axially in each of said vessels within saiddraft tube, whereby on actuation of said agitator the slurry in each ofsaid vessels will flow from the bottom portion of said draft tube upthrough the draft tube and on exiting the top of the draft tube, theslurry will flow downwardly in the space between said draft tube and theinner wall of the vessel.

In the system, said first vessel can be a dissolver vessel foressentially dissolving phosphate rock in said first slurry, said secondevacuated reaction vessel being cooled by evaporation and functioning asa crystallizer vessel for crystallizing calcium sulfate hemihydrate insaid second slurry, and including a fourth, filter feed, vessel, and afourth conduit interconnecting said means for withdrawing slurry fromsaid second reaction vessel with said fourth vessel, for conducting saidsecond slurry containing crystallized calcium sulfate hemihydrate andphosphoric acid to said fourth vessel, and an agitator in said fourthfilter feed vessel for maintaining the slurry therein in suspension.

The fourth circuit can include a surge tank, filter means and,preferably there is a rock box in said conduit for removing anyrelatively large rocks in said second slurry.

The system can contain a fifth conduit for conducting slurry containingcrystalline calcium sulfate hemihydrate and phosphoric acid from saidfourth filter feed vessel to said filter means, for filteringcrystalline calcium sulfate hemihydrate from said slurry, and a sixthconduit connecting said filter with said third reslurry vessel forconducting filtrate containing phosphoric acid to said third vessel.

Preferably said second reaction vessel is positioned at an elevationhigher than said first reaction vessel, said second conduit being anoverflow conduit permitting return of said second slurry in said secondvessel by gravity through said second conduit to said first slurry insaid first vessel.

In the system, said means for introducing phosphoric acid and phosphaterock into said first vessel can comprise an inlet pipe connected withthe interior of said draft tube, and a vent connected to said inlet pipeto permit escape of gases and reduce foaming generated by the dissolvingreaction in said first vessel.

The system can include a sparger in the bottom portion of said secondvessel below the draft tube therein, said means for introducing sulfuricacid into said second vessel comprising an inlet, said inlet beingconnected to said sparger. Preferably the sparger directs the acid tothe bottom of the vessel.

The system preferably includes a first recirculation conduit forselectively recirculating said first slurry from said first vesselexternally thereof and back to said first vessel, a first pump in saidfirst recirculation conduit, a second recirculation conduit forselectively recirculating said second slurry from said second vesselexternally thereof and back to said second vessel, a pump in said secondrecirculation conduit, and valve means for discontinuing slurry flow insaid first conduit from said first vessel to said second vessel duringrecirculating of slurry through said first recirculation conduit or saidsecond recirculation conduit.

The present invention comprises an apparatus which can be useful in theprocess and system described herein. The apparatus useful for thepreparation of phosphoric acid from phosphate rock and a strong acid,which comprises a reaction vessel, a draft tube, means connected to theinner wall of said vessel and mounting said draft tube in a verticalposition within said vessel, said draft tube having an outwardly flaredlower skirt portion terminating in the bottom portion of said vessel, anagitator positioned within said draft tube, a shaft for said agitatormounted axially of said vessel and extending into said draft tube, aninlet conduit to said vessel for introducing a particulate solid and areactive liquid or solvent or a feed slurry (e.g., of phosphate rock andstrong acid) into said vessel, said inlet conduit having a lower endportion terminating within said draft tube, preferably, if the vessel isclosed, a vent pipe connected to said inlet conduit to reduce foaminggenerated by the reaction in said vessel, and means for introducing afluid under pressure (e.g., air) to the bottom portion of said vesseland/or said draft tube skirt.

In another apparatus, which is useful in the present invention, saidinlet conduit has an elongated portion extending downwardly within saidvessel, said inlet conduit having an external upper portion and an inletportion connected to said upper portion, said feed slurry being fed intosaid inlet portion, and said upper portion of said inlet conduit beingsaid vent pipe.

These features and others of the above described apparatus areillustrated in the accompanying FIG. IV.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. I is a cross section of a dissolver vessel illustrating the use offluid conduits for preventing settling of solids within the dissolvervessel;

FIG. II is a schematic of a preferred embodiment of the apparatus andsystem;

FIG. III illustrates a schematic of a preferred inter- and intra-vesselflow pattern;

FIG. IV illustrates a schematic of another embodiment which can be usedfor phosphoric acid manufacture by a gypsum type process;

FIG. V is a photograph of the preferred working embodiment of apparatusshowing fluid conduits between the draft tube and wall of the reactionvessel; and

FIG. VI illustrates the fluid conduits and supports within the reactionvessel for said fluid conduits.

BACKGROUND AND FURTHER DESCRIPTION

The invention is especially directed to the production of phosphoricacid by the calcium sulfate hemihydrate process. In the presentinvention, compared to the prior art "gypsum processes", the control ofreactant concentrations and filtration rates are improved, aconcentrated phosphoric acid (about 35% to about 55% P₂ O₅) is produced,sulfuric acid usage is reduced and a substantial reduction in electricaland heat energy consumption is realized.

Phosphoric acid has been prepared by the wet process for many years. Thewet process involves the reaction of phosphatic solid materials,hereinafter termed phosphate rock, wherein calcium sulfate, monocalciumphosphate, phosphoric acid and sulfuric acid comprise the usual reactionmedia. The names of the three processes for the production of phosphoricacid by the wet process are based on the by-product calcium sulfateproduced; namely, the gypsum or dihydrate process, the hemihydrateprocess, and the anhydrite process. In all such processes the presentinvention can be useful in the reaction of phosphoric acid and phosphaterock. The type of by-product is dependent upon the temperature of thesystem and the P₂ O₅ concentration of the liquid phase of the slurry.Other factors such as fluorine concentration, alumina concentration, andsulfuric acid concentration play a less important role.

As is frequently illustrated by composition diagrams (plottingtemperature versus acid strength), gypsum, (CaSO₄,2,H₂ O) is theby-product formed when the wet process is run at a temperature of about90° C. or less and a P₂ O₅ concentration of up to about 30% in theliquid portion of the slurry. Increasing the temperature to about80°-120° C. and the P₂ O₅ concentration to about 40% in the liquid phasewill yield hemihydrate, CaSO₄ 1/2H₂ O. Adjusting the temperature and theconcentrations, for instance, to 75° C. and 40% P₂ O₅ results in amixture of gypsum and hemihydrate which is very unstable. An unstablesystem such as this causes trouble during filtration due to thehardening or setting up of the gypsum-hemihydrate solid on the filter.Care must be exercised in maintaining the proper temperature and P₂ O₅concentrations in the process being run in order to avoid such problems.CaSO₄ anhydrite is produced at temperatures of about 130° C. and P₂ O₅concentrations greater than 30%. This latter process is most difficultto run due to severe corrosion at the higher temperatures and theinstability of the anhydrite during processing.

It should also be noted that the use of a sodium organosulfonate crystalmodifier in the process system and apparatus herein does not cause anadverse increase in viscosity (contrary to what one would predict fromthe prior art). In fact, with an organosulfonate crystal modifier, theviscosity can be decreased, as is reported by Sikdar and Ore' in AIChEJOURNAL (Vol. 23, No. 3), pages 380-382, May, 1977.

Mixing is critical to filterability of the hemihydrate and a preferredpropeller blade is of the airfoil or hydrofoil design to reduce shearand vortex formation (e.g. those marketed by Mixco). Thorough mixing isvery desirable, whether running the dihydrate, the hemihydrate or theanhydrite process. Good mixing will decrease the localized highconcentration of the reactants, namely, the calcium phosphate and thesulfuric acid. Decreasing such localized concentrations, results in alowering of the substitution losses, a lowering of losses due to coatingthe rock and an improvement in the crystallization conditions.

However, if the mixing involves very high shear, the desirable formationof large, "raspberry" hemihydrate may not occur. It is possible that the"raspberry" habit (as illustrated in FIG. V of Ser. No. 866,963) may beformed by growth around the needle like projections of the "jacks" habit(illustrated in FIG. VI) and that continued high shear cause theseprojections to break off to form "needles" (as illustrated in FIG. VII).It is also possible that improper mixing is a cause for the formation ofsmall fine crystals which are very difficult to filter.

Thus, it is observed that a change of one variable may favorably affectthe recovery of P₂ O₅ from phosphate rock employing one of the wetprocess methods and it may be detrimental to the recovery of P₂ O₅employing a different process. Therefore it is necessary to choose thecombination of process variables which will result in the best recoveryof P₂ O₅ from the phosphate rock along with acceptable filterability ofthe resulting slurry for the process at hand.

The recovery of the phosphate values from the phosphate rock can begreatly increased if the agitation or mixing is maintained at a highlevel. Previous workers in the field have directed their energy toachieve maximum mixing in the wet process. As a result of this activity,today there are one vessel and multi-vessel systems in use for theproduction of phosphoric acid by the wet process. The purpose is toachieve maximum mixing so as to increase the recovery of the phosphatevalues from the phosphate rock and to have the best environment fordissolution of the rock and for crystallization of CaSO₄.

In a one vessel process, the phosphate rock and the sulfuric acid areadded to the slurry in one tank. Agitators, in union with baffles, areused to circulate the slurry into which the reactants (phosphate rockand sulfuric acid) are added. To the extent that the localizedconcentration differences are minimized, the slurry has only one sulfatelevel. This is undesirable, since the phosphate rock should preferablybe dissolved at a low sulfate concentration (preferably negative)whereas crystallization should occur at a high sulfate concentration.

A multi-vessel system can be of two types. Two or more compartments orcells can be constructed within one vessel, the compartments beinginterconnected in series. The reactants are added separately, that is,in different compartments in order to increase the dispersion of saidreactant in the slurry prior to reacting with the other reactant. At thelast compartment, some slurry is removed from the system for recovery ofphosphoric acid; the major portion of the slurry being recycled to thefirst compartment.

Multi-vessel processes involve the use of two or more vessels connectedin series, the reactants are added to the slurry in separate vessels soas to more completely disperse one reactant in the slurry before it iscontacted by the later added reactant(s). Again the system is arrangedso that a portion of the slurry is recycled from a later reactor back tothe first reactor.

Air spargers of the present invention can be useful in many prior artprocesses and apparatus.

For example, Caldwell, U.S. Pat. Nos. 3,415,889 and 3,939,248 andBergstrom, U.S. Pat. Nos. 3,666,143 and 3,917,457 developed acombination reactor-cooler which is fitted with a draft tube. The vesselwas maintained under a vacuum while the slurry was circulated within avessel. Using the draft tube with an agitator it is possible tocirculate the slurry at such a flow rate that upwards of 200% of thevolume of the slurry is circulated through the draft tube per minute,constantly renewing the surface of the slurry exposed to the vacuum.With this type of circulation, dispersion of the reactants is improvedover the conventional one vessel system. In addition to betterdispersion of the reactants, the slurry on exposure of the vacuum at thesurface is cooled by evaporation of water. The temperature differentialwithin the system is minimized by the rapid flow rate realized. Thecooled slurry is immediately mixed with the hot slurry minimizing thelocalized cooling affect.

Lopker, U.S. Pat. Nos. 3,522,003 and 3,522,004 describes the use in agypsum process of a two vessel system for the production of phosphoricacid from phosphate rock and sulfuric acid. A vacuum is applied to onevessel and cools the slurry by evaporation of water. The cooled slurryis then rapidly dispersed within the system minimizing cooling effectsand preventing supersaturation of the calcium sulfate due to reducedtemperatures. The levels of the slurries within the two vessels arevertically offset.

Fitch (U.S. Pat. No. 3,553,918) describes a process for the productionof concentrated phosphoric acid and gypsum including the acidulation ofphosphate rock in a first zone in which the resulting slurry containsfrom about 1% (-2.45% SO₄.sup.═) to about 4.5% (-11% SO₄.sup.═) excesscalcium. The slurry produced in the first zone is then transferred to asecond zone in which an excess of sulfuric acid is present such thatfrom about 3% to about 6% excess sulfuric acid is present in the slurry.Hemihydrate initially produced is converted to gypsum.

Peet (U.S. Pat. No. 3,885,263) describes an anhydrite process and Long(U.S. Pat. No. 3,453,076), Peet (U.S. Pat. No. 2,885,264) and Robinson(U.S. Pat. No. 3,418,077) describe processes for the production ofphosphoric acid by the hemihydrate process. No additionalrecrystallization of the CaSO₄.1/2H₂ O is required in these hemihydrateprocesses. In the Robinson process phosphoric acid containing from about40% to about 55% P₂ O₅ by weight is produced.

This process comprises in a first stage reacting in the presence ofexcess calcium ions, phosphate rock with at least nine parts by weightof phosphoric acid for each part of calcium added, said phosphoric acidcontaining at least 37% by weight P₂ O₅ and 1% to 3% by weight dissolvedsulfate, whereby the phosphate rock is converted into a slurrycomprising monocalcium phosphate, phosphoric acid, and calcium sulfate,the percentage of calcium ion precipitated as calcium sulfate being 10to 60%, preferably 20-50% by weight of total calcium fed, in a secondstage reacting the slurry from the first stage with sulfuric acidwhereby phosphoric acid containing at least 40% P₂ O₅ by weight andcalcium sulfate hemihydrate is formed, the sulfuric acid being used inan amount 0.5 to 2.0% by weight in excess of that required to convertthe calcium content of the phosphate rock fed to the first stage intocalcium sulfate. In the third stage, the phosphoric acid is separatedfrom the calcium sulfate and the crystals are washed. The temperature ofthe first and second stages are in the range from 80° to 111° C.,preferably from 90°-110° C.

DETAILED DESCRIPTION

This invention is useful in a process for the production of phosphoricacid wherein phosphate rock is reacted or "dissolved" in phosphoric acid(but can be useful where nitric acid is used).

A major benefit of the present invention is that the stream or slurry(comprising phosphoric acid and calcium sulphate hemihydrate) which ispassed through the separation section (e.g. a filter), can have abeneficially low content of fines (i.e. solids with an average particlediameter less than about 5 microns) and a low viscosity, especially whenan organic sulfuric acid or organic sulfuric acid salt is added as acrystal modifier at a concentration in the range of 1-1000 ppm, morepreferred 5-100 ppm, by weight based on the total weight of the slurrytransferred to the separation section.

For production of phosphoric acid in a large scale commercial plant, itis essential that the stream to the filter have a high filtration rate,preferably at least 0.2 tons P₂ O₅ /day ft² (more preferred at least0.5).

The filter rate can be calculated as shown in the applications of Ore etal, filed Dec. 29, 1977.

Phosphate rock, either calcined or uncalcined, and phosphoric acid areadded to a first slurry comprising phosphate rock, calcium sulfatehemihydrate, monocalcium phosphate, phosphoric acid and sulfuric acid.Preferably, the phosphate rock is slurried in the phosphoric acid priorto the addition to the first slurry. Phosphate rock, about 95% of +100mesh, containing at least 32% P₂ 0₅ is the preferred source of phosphatefor the process. Ground or unground rock can be used, For example,phosphate rock of 95% of -200 mesh can be used. Rocking containing lessthan 32% P₂ O₅ is acceptable, and can be employed in this process. Highalumina phosphate pebble may also be used, especially when the resultingacid is purified by the process of U.S. Application Ser. No. 676,559filed Apr. 13, 1976 of Ore, the entire disclosure of which is herebyincorporated herein. For hemihydrate, the phosphate rock is slurried inphosphoric acid that contains from about 20% to about 40% P₂ O₅.Phosphoric acid, recycled from the separation section, containing fromabout 20% to about 40% P₂ O₅ (and usually some sulfuric acid) is usuallyused in the process. When the phosphoric acid is recycled from theseparation section it will usually contain from about 0.5% to about 3.5%sulfuric acid by weight. Phosphoric acid from other sources, such asother phosphate plants, merchant grade acid may be used.

For hemihydrate, the temperature of the phosphate rock-phosphoric acidmixture is maintained at about 50° C. to about 100° C., preferably fromabout 90° C. to about 100° C. The resulting mixture is from about 30% toabout 40% solids by weight, about 33% being preferred. A defoamer isadded if and when required. Calcination of the rock can reduce oreliminate foaming. Various antifoam agents can be used, including talloil, tall oil fatty acids, alkyl esters and part esters of tall oilfatty acids, sulfated tall oil fatty acids, tall oil rosin, alkoxideadducted tall oil rosin, oleic acid, sulfated oleic acid, silicones,reaction products of amines and carboxylic acids and mixtures of two ormore of such defoamers.

The phosphate rock-phosphoric acid mixture is added to a first slurry ofcalcium sulfate hemihydrate, phosphoric acid, monocalcium phosphate andsulfuric acid in a first reaction vessel. The phosphate rock andphosphoric acid may be admixed in a separate vessel or added separatelyto the first slurry in the first reaction vessel. The phosphaterock-phosphoric acid mixture on being added to the first slurry in thefirst reaction vessel is rapidly dispersed within the first slurry. Afirst portion of the first slurry is transferred to a second reactionvessel.

The first reaction vessel is fitted with a draft tube and an agitator(although the draft tube can, in some cases, be removed). The agitatorcan consist of a shaft fitted with a propeller at the bottom thereof.The agitator is so located with respect to the draft tube that onactivation of the agitator, a second portion of the first slurry isdrawn from the bottom of the draft tube up through the draft tube andout the top of the draft tube. On exiting the draft tube said slurrypasses in a downward direction in the space between the draft tube andthe walls of the first reaction vessel. The direction of circulationthrough the draft may be reversed and is not critical. In this first ordissolver vessel (or set of vessels), considerable gas (CO₂) isgenerated and, like a giant milkshake, the apparent density of thecontents can be about 1.0, although when the gas is removed, the actualdensity of the contents is about 1.6 to 2.0.

Circulation is thus established within the first reaction vessel. Therate at which said slurry is circulated is at least equal about 50% ofthe volume of the slurry in the first reaction vesel per minute,preferably from about 50% to about 150% and most preferably about 100%.This circulation thoroughly disperses the phosphate rock-phosphoric acidmixture within the first slurry. The recycle phosphoric acid dissolvesthe P₂ O₅ in the rock forming monocalcium phosphate. This is anexothermic reaction which suplies the heat required to maintain thetemperature of the slurry in the first reaction vessel between about 66°C. to about 113° C. The soluble or "excess" sulfate content of the firstslurry is typically maintained at about +0.0% to about -4%, (but,depending on the method of sulfate analysis, the soluble sulfate can beas low as -7, below which the rock dissolution practically stops due tosaturation of calcium phosphate). The first slurry, whether in one or aplurality of reactors is maintained at an excess of Ca⁻² (a deficiencyof SO₄ ⁻²) for stoichiometric formation of CaSO₄.

As measured, soluble sulfate values can be either positive or negative.Soluble sulfate values include not only the sulfuric acid present in theliquid component of the slurry but also the soluble calcium sulfatethere present. Negative soluble sulfate values indicate that excess ofcalcium ions are present in the solution, as is usually observed in thephosphate rock-phosphoric acid mixture. Positive soluble sulfate valuesindicate that excess sulfate ions are present. A value of 0.0% indicatesthat the sulfate ions and the calcium ions are equivalentstoichiometrically within the limits of the analysis.

One typical analysis is 0.9% CaO and 2.2% SO₄ which would calculate##EQU1## This leaves 2.2-1.5=+0.7% "free or soluble" sulphate. That is,a positive soluble sulphate.

Another analysis is 0.98% CaO and 1.4% SO₄, which calculates ##EQU2##That is, there is insufficient sulphate concentration to combine withall of the calcium, which is reported as a negative value.

As is described further in the application of Chemtob, filed of evendate herewith and incorporated herein by this reference, the numericalvalue of negative sulfate can vary somewhat depending on the analyticalprocedure for sulfate ions.

For positive sulfate values, there is little or no difference betweenvalues obtained by different analytical methods. The preferred method ofcalcium analysis is by atomic absorption, which is highly accurate forboth positive and negative sulfate.

The residence time of the solids in the first reaction vessel is fromabout 2.0 hours to about 5.0 hours, preferably from about 2.5 hours toabout 4.5 hours.

A first portion of the first slurry is transferred through a firstconduit into a second reaction vessel. The second reaction vessel, whichis preferably subjected to a vacuum, is fitted with a draft tube, anagitator and a sulfuric acid inlet. The agitator consists of a shaftfitted with a propeller at the bottom thereof. The shaft and agitatorare so located with respect to the draft tube that on actuation of theagitator a second portion of the second slurry is caused to flow fromthe bottom of the draft tube up through the draft tube and out the topof the draft tube. On exiting the draft tube, said second portion of thesecond slurry flows in a downward direction in a space between the drafttube and the inside walls of the second reaction vessel. The directionof the circulation can be reversed and is not critical. The rate atwhich the slurry is circulated is at least equal to about 50% of thevolume of the slurry in the vessel per minute, preferably from about 50%to about 150% of the volume and most preferably about 100 % of thevolume.

Sulfuric acid, preferably about 98%, is added through the sulfuric acidinlet into the second slurry either as is or mixed with phosphoric acid.The first portion of the first slurry is also added into the secondslurry.

A crystal modifier, usually a derivative of tall oil or of an organicsulfonic acid, preferably a salt, can be added to the slurry in thesecond reaction vessel. The crystal modifier can also be added to thefirst reaction vessel. A preferred crystal modifier is selected fromalkyl, aryl, alkylaryl, and alicyclic derivates of sulfonic and sulfuricacids in which the organic radical contains from about 12 to about 30carbon atoms. The free acid, salts thereof and mixtures of the free acidand salts may be used. The preferred salts include those of alkalimetals, ammonia and alkyl, aryl or alkylaryl amines (e.g. trimethylamine, diethyl amine, monopropyl amine). Polymeric sulfonates andsulfates can also be employed. Examples of crystal modifiers which canbe employed in the present process are alkyl sulfonic acids containingfrom about 12 to about 30 carbon atoms, benzenesulfonic acid,alkylbenzenesulfonic acid in which the alkyl group contains from about 8to 20 carbon atoms, alkylcyclohexane sulfonic acid in which the alkylgroup contains from about 8 to 20 carbon atoms, polymeric sulfonates andsulfates such as polystyrene sulfonate, and polyvinylsulfonate, saidpolymeric material having a molecular weight of from about 500 to about1,000,000. The organic sulfonic acid can be an alkyl-, aryl-, or analkylaryl-sulfonic acid or a sulfated derivative of a carboxylic acid oran alkalimetal, amine or ammonium salt thereof.

For example, tetradecylsulfonic acid, benzene-sulfonic acid,isooctylbenzene sulfonic acid and sulfated oleic acid may be used ascrystal modifiers in this process. Mixtures of two or more modifiers arealso useful. The crystal modifier is added for the purpose of increasingthe growth of the hemihydrate crystals formed in the system. Thepreferred salts include those of sodium, potassium, ammonia and primary,secondary and tertiary alkyl amines containing from 1 to about 30 carbonatoms. Preferably, the modifier as described above, is present at alevel of about 1 to 1000 ppm (usually 5 to 500 ppm) based on the weightof slurry to the separation section. Preferably, the levels of modifierand of defoamer are kept as low as possible (while maintaining goodfilterability), since residual quantities in the phosphoric acid productcan cause crud formation (e.g., emulsions and deposits) if the acid islater treated to remove magnesium impurities by the process of U.S.Patent application Ser. No. 688,265 filed May 20, 1976 now U.S. Pat. No.4,053,564 issued Oct. 11, 1977, reissue Ser. No. 83,890 filed Oct. 11,1979 (which is incorporated herein) and the related processes in U.S.Ser. No. 840,791 filed Oct. 11, 1977.

An unexpected discovery is that salts of sulfonic acids can be used ascrystal modifiers in the process of the present invention withoutcausing an adverse increase in solution viscosity. This result issurprising, in view of the teaching in Slack, Phosphoric Acid, M.Deckker, Inc., New York, 1968 (at page 383), "Neutral surfactants, suchas a salt of ABS, have an adverse effect on slurry viscosity".

The flow of the second slurry within the second reaction vesselthoroughly disperses the first portion of the first slurry, the sulfuricacid and the crystal modifier within the second slurry. The location ofthe sulfuric acid inlet in the second reaction vessel is not critical.It may be at the top, the middle, the bottom or at intermediate pointsof the second reaction vessel. The sulfuric acid conduit attached to thesulfuric acid inlet may enter the second reaction vessel from the top,the bottom, or points intermediate therein, the exact point of entranceinto the vessel is not critical.

Phosphoric acid, if needed, can be added to the second slurry within thesecond reaction vessel.

The surface of the second slurry in the second reaction vessel ispreferably exposed to a pressure of between about 2 to about 29 inchesof mercury absolute, more preferably from about 3 to about 20 inchesmercury absolute. Water and volatile components added to, or producedin, both the first and second slurries can be removed from the secondslurry by evaporation causing a reduction in the temperature of thesecond slurry. The cooled second slurry is thoroughly mixed so thattemperature differentials are minimized within the total volume of thesecond slurry. With this evaporative cooling, the temperature of thesecond slurry is maintained between about 66° C. to about 113° C.preferably from 80° C. to about 105° C. Although it is greatlypreferrable to operate the second vessel under reduced pressure, theprocess can be run while maintaining both the first and second reactionvessels at atmospheric pressure. Sulfuric acid, which is added to thesecond slurry in the second reaction vessel through the sulfuric acidinlet, can be from about 89% to 99% H₂ SO₄, typically about 98% H₂ SO₄.

The total sulfate value added to the system is the sum of the sulfatevalues in sulfuric acid added plus the sulfate values added in the rock.Surprisingly in the present process this total can be only about 90% to100% of the stoichiometric amount of sulfate needed to convert thecalcium added in the rock fed to the first reaction vessel into calciumsulfate hemihydrate.

The soluble sulfate content as measured in the second slurry should befrom about +0.7% to about +4.5%, preferably from about 1.5% to about3.0%. The specific gravity of the slurry in the second reaction vesselis about 1.80%±0.2 g/cc. The specific gravity of the liquid portion ofthe slurry is about 1.56±0.20 g/cc. The liquid gravity corresponds to aphosphoric acid which contains about 44%±10% P₂ O₅. Residence time ofthe solids in the second reaction vessel is from about 0.6 hour to about2.0 hours, preferably from about 0.7 hour to about 1.6 hours.

A first portion of the second slurry flows from the second reactionvessel back to the first reaction vessel through a second conduit and isthoroughly dispersed within the first slurry.

It is the flow of the second slurry to the first slurry which aids incontrolling the temperature of the first slurry and adds sulfate values(sulfuric acid) and phosphoric acid values to the first slurry in orderto dissolve the rock. Additional sulfate values are usually added to thefirst slurry in the first reaction vessel with the recycled phosphoricacid. Circulation between vessels and within vessel minimizes localizedconcentration of reactants of hot slurry and of cooled slurry thusresulting in a more easily controlled process than previously observed.

A third portion of the second slurry is removed from the second reactionvessel and is transferred through a conduit to a reservoir. The thirdportion of the second slurry, on a weight basis, is approximately equalto the phosphate rock, the phosphoric acid, and the sulfuric acid addedin the first and second reaction vessels respectively minus thevolatiles (on a weight basis) removed from the second reaction vesselwhich can be subject to a vacuum. The third portion of the second slurryis treated for recovery of the phosphoric acid and calcium sulfatecontained therein.

For plant control purposes, the flow rates of the reactants and of theslurries can be adjusted in accordance with the analytical valuesobtained in order to maintain the desired sulfate levels within thereaction system. It is to be understood that the system described can berun on a batch or continuous basis (wherein the reactants can becontinuously added and the third portion of the second slurry can becontinuously removed from the system prior to separation into phosphoricacid and calcium sulfate hemihydrate).

A defoamer is added if and when required. The defoamer may be selectedfrom the group consisting of tall oil rosin, alkoxylated tall oil rosin(see U.S. Pat. No. 3,594,123, issued July 20, 1971 to Encka et al), talloil fatty acids, whole or part esters of tall oil fatty acids, sulfatedtall oil fatty acids, alkoxylated tall oil fatty acids, oleic acid,sulfated oleic acid, silicones and mixtures of a monocarboxylic acid(12-22 carbon atoms) and monoalkanoylamide derivatives of themonocarboxylic acid. The preferred defoamer is a mixture of methylesters of tall oil fatty acid and tall oil fatty acids sold by AZProducts Co. of Eaton Park, Fla. under the tradename "AZ 10A" (becauseAZ 10A also acts as a crystal modifier). The crystal modification is notdue entirely to the presence in AZ 10A of a sodium alkyl sulfonatebecause crystal modification also occurs when the sulfonate is removed.The amount of the defoamer used is preferably from about 0.01% to about0.3% (typically 0.04 to 0.1) by weight based on the weight of the slurrytransferred to the separation section (or about 0.05% to 1.5% based onP₂ O₅ produced by the process). As is noted hereinafter, venting of thereslurry and/or dissolver vessels can reduce defoamer usage.

AZ 10A is further discussed in the applications U.S. Ser. Nos. 865,556and 865,557 of Ore et al, filed Dec. 29, 1977.

One of the preferred crystal modifiers, especially when AZ 10A is alsopresent, is Actrasol W-40. Actrasol W-40 is a mixture of predominantlysaturated sodium alkyl sulfonates. The alkyl groups are in the 12 carbonranges, although there is a distribution from about 9-15 carbons. Thereare approximately 16 different sulfonates in the mixture; undoubtedlymany are isomers and homologs of each other. It appears that ActrasolW-40 is made by the sulfonation of propylene tetramer, butylene trimer,or other material consisting of a mixture of isomers and homologs.Actrasol W-40 is further described in the application of Ore et al filedDec. 29, 1977.

The P₂ O₅ yield of the process can be improved by converting thehemihydrate filter cake to dihydrate cake by repulping it in water andsulfuric acid. The dihydrate slurry is filtered to separate the aqueousphase (phosphoric acid 30-35% P₂ O₅) which is recycled to rockdigestion.

There are three analytical procedures which can be used to determinesulfate in phosphoric acid; namely, gravimetric, titration andturbidity. These are all further described in the application ofChemtob.

With some solutions, where negative sulfate ion is concerned, thetitration method can give higher values than the turbidity method (e.g.,-1.0% sulfate by titration and -2% by turbidity) or -3.3% by titrationversus -6% by turbidity). However, both methods are usually in closeagreement where positive sulfate is determined.

In the hemihydrate process, the most important factor in the preferredoperation is that by about every analytical method, a negative sulfatebe maintained in the first (dissolver) vessel. Even a slight positivesulfate (e.g. +9.7%) in the dissolver can cause decreases in yield ofphosphoric acid produced by the process (one cause being due to greatlyincreased nucleation, another at about 1.5% SO₄ to coating of thephosphate rock which decreases the amount dissolved).

Mesh size of the rock also influences yield, the smaller the particlesize, the better the yield. Especially good results are obtained by wetscreening rock to -28 or smaller. No drying is needed.

If the second (crystallizer) reactor is not subjected to reducedpressure to provide cooling, external cooling means (such as a flashcooler) can be provided to cool the slurry.

DETAILED DESCRIPTION OF THE DRAWINGS

FIGS. I, II, and III are schematic diagrams of embodiments of theapparatus. In FIG. III, phosphoric acid at about 70° C., which is addedthrough conduit 6, and phosphate rock, which is added through conduit 8,are slurried in vessel 2 which is fitted with an agitator 4. Defoamercan be added as needed through conduit 10. In a preferred embodiment,conduit 10 is of a much greater height and/or diameter than is requiredsolely for introduction of the defoamer and, thus, can function as avent. Properly chosen venting can greatly reduce foaming and, in manycases, can eliminate the need for a defoamer in the "reslurry" vessel 2.The temperature of slurry 11 so formed is about 92° C. and the solidscontent is about 30% to about 40% by weight. Slurry 11 is transferredthrough conduit 12 to vessel 16. Vessel 16 is fitted with an agitator(shaft 18 and propeller 21 attached to the bottom thereof), and a drafttube 20 which is secured to the inside wall of vessel 16 by braces (notshown). Slurry 11 flows into slurry 22 which is composed of calciumsulfate hemihydrate, monocalcium phosphate, phosphoric acid, andsulfuric acid. The propeller 21 of the agitator is so positioned withrespect to the location of the draft tube 20 that on actuation of theshaft 18 and propeller 21 by a motor (not shown), slurry 22 in vessel 16will flow from the bottom portion of the draft tube 20 up through thedraft tube. On exiting the top of the draft tube, slurry 22 will flowdownwardly in the space between the draft tube 20 and the inside wallsof vessel 16. Alternatively, the propeller blade can be positionedcloser to the bottom of the draft tube and, if desired, the flow can bereversed (i.e., to flow upwardly in the space between the draft tube andthe inside walls). A first portion of slurry 33 is transferred fromvessel 28 through conduit 38 to vessel 16. The flow created withinvessel 16 thoroughly mixes slurry 11 and slurry 33 within slurry 22.Slurry 22 is then transferred to vessel 28 through conduits 24 usingpump 25. Vessel 28 may be vertically offset from vessel 16 or it may beon the same level as vessel 16. Samples for analysis of the first slurryare removed from sample port 24a. Slurry 22 is at a temperature of about66° C. to about 113° C., and has a soluble sulfate value of about +0.7to about -4% (most preferred below 0.0).

On entering vessel 28 which is equipped with an agitator (shaft 30 andpropeller 31 attached to the bottom thereof), a draft tube 32 and asulfuric acid inlet 34, slurry 22 is dispersed into slurry 33. Drafttube 32 is secured to the inside wall of vessel 28 by braces (notshown). Sulfuric acid is added from the sulfuric acid inlet 34 and isalso thoroughly dispersed into slurry 33. Crystal modifier may be addedto vessel 28 through an inlet 23a. Activation of the agitator (shaft 30and propeller 31) by means of a motor (not shown) causes a flow ofslurry 33 from the bottom of the draft tube 32 up through the draft tubeand out the top portion of said draft tube. On exiting the top of thedraft tube 32, the slurry flows downwardly in the space between thedraft tube 32 and the inside walls of vessel 28. A circulationestablished within vessel 28 disperses slurry 22 and sulfuric acid intoslurry 33, constantly renewing surface 36. Vessel 28 is maintained at apressure of about 2 inches of mercury to about 29 inches of mercuryabsolute. Water is evaporated from the hot slurry, thus cooling theslurry. In addition to water, other volatile materials produced by thereaction of sulfuric acid and phosphate rock are also removed. Thesematerials include CO₂, HF, SiF₄, H₂ S, SO₂ and others. Because of theinternal circulation of the slurry within vessel 28, temperaturegradients are minimized. Slurry 33 (maintained at a temperature of about66° C. to about 113° C., preferably from about 80° C. to about 105° C.,and having a positive sulfate content, preferably about +0.7 to about+4.5%) is recirculated back to vessel 16 through conduit 38. Slurry 33is efficiently dispersed within slurry 22 in vessel 16 by means of theinternal circulation within vessel 16. Thus, a system has been developedin which both inter and intra-vessel circulation occur so as to betterdisperse the reactants being added to the slurries and to reducetemperature gradients within the vessels due to heating and cooling.This circulation system permits rapid and easy plant control and thenegative sulfate level in the dissolver acts like a buffer to adsorbinadvertent sulfate concentration increases in the crystallizer.

A portion of slurry 33 about equal to the amount of reactants added(phosphoric acid, phosphate rock an sulfuric acid), minus the amount ofwater and volatiles removed from the system is removed from vessel 28through conduit 40. Samples for analysis of the second slurry areremoved from sample port 42 located on conduit 40. The slurry is pumped(pump 35) to the diversion or splitter box 44 from which it flows tovessel 48 through conduit 46. Agitator 50 mainatins the slurry in adispersed condition in vessel 48. The slurry is pumped (pump 3a) fromvessel 48 through conduit 52 to the separation section (not shown inFIG. III but shown in FIG. II). In start-up, valve 52a can be used todirect the slurry through line 3 to vessel 2, and, thus, bypass thefilter and provide a fast means of building up the solid level in theslurry without dumping wet pans.

Reactants are continuously added to vessel 16 and 28, while water andvolatiles and the product slurry are constantly being withdrawn fromvessel 28. In case of a separation apparatus breakdown, the system canbe placed on recycle. No reactants are added to the system. Intra-vesselcirculation would continue and inter vessel circulation would bediscontinued. As is discussed further hereinafter, the circulationsystem in FIG. III is especially useful in providing an economical andeasy to control start-up or restart (after a short interruption)procedure.

The dissolver apparatus 16 contains two sets of air spargers 82, 83(c.f. FIG. I) in the two quadrants furthest away from the feed inlet 38aof FIG. I.

It is to be recognized that the elevation of vessels 2, 16, 28, 44 and48 with respect to each other is a preferred arrangement but may bevaried. Likewise, the conduits connecting vessels 2, 16, 44 and 48 maybe rearranged, additional conduits added and/or existing conduitsdeleted. For example, slurry 22 passing from vessel 16 to vessel 28 maybe introduced into the top part of vessel 28 rather than the bottompart.

Instead of adding the reactants phosphoric acid, phosphate rock and ifnecessary, the defoamer to a preslurry vessel 2 as shown in FIGS. I andIII, the reactants can be added directly to the first slurry 22 invessel 16. The phosphoric acid is added through a provided conduit andthe phosphate rock is added through another provided conduit. Thereactants are added in amounts such that the direct combination of thetwo results in a slurry containing between about 30% to about 40% solidsby weight and an initial concentration of about 13% to about 47% P₂ O₅in the liquid portion of the slurry. Defoamer is added through conduit13, if, and when needed. Once the reactants are dispersed in the firstslurry 22, the parameters such as temperatures, pressures,concentrations, and flows are the same as described above for the morepreferred embodiment; however, a greater number of air spargers may berequired.

FIG. III also illustrates a system and apparatus for phosphoric acidproduction by calcium sulfate hemihydrate formation. This system for thepreparation of phosphoric acid from phosphate rock and sulfuric acidincludes in combination a first reaction vessel (the dissolver, 16)containing a first slurry (22) comprising calcium sulfate hemihydrate,monocalcium phosphate and phosphoric acid, a second reaction vessel (thecrystallizer, 28) containing a second slurry (33) comprising calciumsulfate hemihydrate, monocalcium phosphate, sulfuric acid and phosphoricacid, means in each of said vessels for maintaining a continuouscirculation of the slurry therein, said last mentioned means including adraft tube (20 in the dissolver, 32 in the crystallizer) disposedcentrally in each of said vessels and an agitator (18, 30) positionedaxially in each of said vessels within said draft tube, whereby onactuation of said agitator the slurry in each of said vessels will flowfrom the bottom portion of said draft tube up through the draft tube andon exiting the top of the draft tube, the slurry will flow downwardly inthe space between said draft tube and the inner wall of the vessel, afirst conduit (24) interconnecting said first and second reactionvessels for conducting said first slurry from said first reaction vesselto said second reaction vessel, a second conduit (e.g., an overflow 38)interconnecting said vessels for conducting said second slurry from saidsecond reaction vessel to said first reaction vessel, means for applyinga vacuum (as indicated at 28a) to said second reaction vessel to effecttemperature control in said second reaction vessel and to thereby form avacuum seal between said first and second reaction vessels, an inletpipe (12) for introducing phosphate rock and phosphoric acid to saidfirst reaction vessel, said inlet pipe connected with the interior ofthe draft tube in said first vessel, means for introducing sulfuric acidto said second reaction vessel (e.g. a sparger 34) and means (40) forwithdrawing a slurry containing phosphoric acid and calcium sulfatehemihydrate from said second reaction vessel.

The system can include a third reslurry vessel (2) for reslurryingphosphate rock and recycle phosphoric acid, and a third conduit (12)interconnecting said third vessel with said inlet pipe to said firstreaction vessel. In the system, the first vessel can be a dissolvervessel for essentially dissolving phosphate rock in said first slurry,said second evacuated reaction vessel being cooled by evaporation andfunctioning as a crystallizer vessel for crystallizing calcium sulfatehemihydrate in said second slurry, and including a fourth filter feedvessel (48), and a fourth conduit (40b and 46) interconnecting said lastmentioned means (40) for withdrawing slurry from said second reactionvessel with said fourth vessel, for conducting said second slurrycontaining crystallized calcium sulfate hemihydrate and phosphoric acidto said fourth vessel, and an agitator (50) in said fourth filter feedvessel for maintaining the slurry therein in suspension. The system caninclude a surge tank (44) or a splitter box (44a in FIG. II) in saidfourth conduit which provides a sufficient liquid head to provide avacuum seal and (with the splitter box) a break in the line to preventsiphoning.

A splitter box is a compartmented vessel, usually containing a wire-likepartition (which can be fixed or movable) and is usually used like avalve in a conduit system for a slurry (that is, it usually is used todivert flow).

The system can include filter means (the filter in FIG. II), a fifthconduit (52) for conducting slurry containing crystalline calciumsulfate hemihydrate and phosphoric acid from said fourth filter feedvessel to said filter means, for filtering crystalline calcium sulfatehemihydrate from said slurry, and a sixth conduit (6) connecting saidfilter means with said third reslurry vessel for conducting filtratecontaining phosphoric acid to said third vessel. The system can includea rock box (see FIG. II) in said fourth conduit for trapping anyrelatively large rocks, stones and objects (especially solids formed onthe sulfuric acid sparger) in said second slurry. The rock box alsoincludes washing means (e.g., a water line, not shown) and dischargemeans (e.g., a drain, not shown) for removing entrapped particles duringa shut down period or they can be shoveled out. In the system, the firstdissolver vessel (16) can be positioned at an elevation higher than saidfourth filter feed vessel (48) and include an overflow pipe (26) fromsaid first vessel to said fourth vessel for conducting overflow slurryfrom said first vessel by gravity to said fourth vessel.

In the system, the second reaction vessel (28) can be positioned at anelevation higher than said first reaction vessel, said second conduit(38) being an overflow conduit permitting return of said second slurryin said second vessel by gravity through said second conduit to saidfirst slurry in said first vessel. In the system as defined in claim 1,including a vent (5) connected to said inlet pipe (12) to permit escapeof gases and reduce foaming generated by the dissolving reaction in saidfirst vessel. The system can include a sparger (34) in the bottomportion of said second vessel below the draft tube therein, said meansfor introducing sulfuric acid into said second vessel comprising aninlet (34a), said inlet being connected to said sparger. The system caninclude a first recirculation conduit (24, 14) for selectivelyrecirculating said first slurry from said first vessel externallythereof and back to said first vessel, a first pump (25) in said firstrecirculation conduit, a second recirculation conduit (40, 40a, 23b) forselectively recirculating said second slurry from said second vesselexternally thereof and back to said second vessel, a second pump (35) insaid second recirculation conduit, and valve means (15,17) fordiscontinuing slurry flow in said first conduit from said first vesselto said second vessel and to permit recirculation of slurry through saidfirst recirculation conduit or valve means (29, 43) to permitrecirculation of slurry through said second recirculation conduit.

FIG. I shows a preferred apparatus (16), useful for the preparation ofphosphoric acid from phosphate rock and a strong acid, which comprises aclosed vessel 62, a draft tube 20, means (braces 64) connected to theinner walls of said vessel and mounting said draft tube in a verticalposition within said vessel, an agitator 21 positioned within said drafttube, a shaft 18 for said agitator mounted axially of said vessel andextending into said draft tube, an inlet conduit 12 to said vessel forintroducing a feed slurry of phosphate rock and strong acid into saidvessel, said inlet conduit having a lower end portion 12a terminatingwithin said draft tube 20.

In the apparatus, the draft tube can have an outwardly flared lowerskirt portion 66 terminating in the bottom portion of said vessel, andcan include a vent pipe 5 connected to said inlet conduit to reducefoaming generated by the reaction in said vessel. The inlet conduit canhave an essentially vertical portion 12b positioned within said vessel,and have an external vertical upper portion 5 extending above saidvessel and an inlet portion 70 externally of said vessel and connectedat an angle to said vertical external upper portion of said inletconduit, said feed slurry being fed into said inlet portion 70, and aninterconnected vertical upper portion 1 of said inlet conduit being saidvent pipe.

In the apparatus, the agitator 21 can be positioned in the upper portionof said draft tube 20 (as in FIG. I), or at the lower portion, or at anintermediate level. The lower end portion 12a of the inlet conduit 12can terminate in the draft tube 20 below the agitator 21. The vessel 16can have an outlet 72 in the lower end thereof below said draft tube 20.

The inner wall 74 of said vessel adjacent to the bottom thereof can bedownwardly dished at 16a adjacent the lower end of said draft tube. Theapparatus can include an overflow pipe 26 extending from the upper endportion of said vessel 62, and a recycle slurry pipe 38 extending intosaid vessel 62 and terminating at a zone in the annulus thereof betweenthe outer wall of said draft tube 20 and the inner wall 74 of saidvessel, adjacent the lower skirt portion 66 of said draft tube.

The apparatus can include means 1a for introducing a treating ordefoaming agent into said vent pipe 1. In the apparatus, the draft tube20 can have an outwardy flared lower skirt portion 66 terminating in thebottom portion of said vessel 62, said vessel having an outlet 72 in thelower end thereof below said draft tube, the inner wall 74 of saidvessel adjacent the bottom thereof being downwardly dished (at 16a)adjacent the lower end 66 of said draft tube 20. The apparatus caninclude an overflow pipe 26 extending from the upper end portion of saidvessel 62, a recycle slurry pipe 38 extending into said vessel andterminating at a zone in the annulus thereof between the outer wall ofsaid draft tube and the inner wall 74 of said vessel, adjacent the lowerend portion of said draft tube, and an additional inlet pipe 68 in saidvessel for the introduction of steam or a treating or defoaming agent.

The apparatus includes means (82,84) for introducing air under pressureto the bottom portion of the vessel or under the lower portion of thedraft tube skirt (83,85).

ILLUSTRATIVE EXAMPLES Example 1

Vessels 16 and 28 and the accompanying connective means such asconduits, pumps, etc. of FIG. II are filled with a slurry consisting ofcalcium sulfate hemihydrate, monocalcium phosphate, phosphoric acid andsulfuric acid. The weight percent of the solids in the slurry is about31%, the specific gravity of the degassed slurry in vessel 28 is about1.80 g/cc. P₂ O₅ concentration of the liquid portion of the slurry isabout 42% by weight. The temperature of the slurry in vessel 16 isbetween about 88°-102° C. preferably between 92° C. and 105° C., whereasthe temperature in vessel 28 is between 88° and 105° C., preferably 92°C. and 105° C. Soluble sulfate concentration in vessel 16 is from about+0.6 to about -7% (preferably 0.0 to -6) and the soluble sulfateconcentration in vessel 28 is above 0.0 (preferably from about 0.7% toabout +4.5%).

A mixture of phosphate rock, and phosphoric acid is prepared by addingphosphate rock to phosphoric acid in the ratio of about 1647 pounds ofphosphate rock (about 31.2 P₂ O₅ and 45.6 CaO) to about 3700 pounds ofphosphoric acid (about 32% P₂ O₅). The temperature of the mixture isabout 90° C. A tall oil-sulfonic acid defoaming agent (AZ 10A) is addedas needed to reduce the foam caused by partial dissolution of thephosphate rock in phosphoric acid.

This phosphate rock-phosphoric acid mixture is added to the first slurryin vessel 16 at the rate of about 380 gpm (about 5350 pounds perminute). The incoming mixture is thoroughly mixed with the first slurryand a first portion of the second slurry from the second reactionvessel. Intra vessel mixing is accomplished by means of the draft tubeand the agitator. The first slurry is pumped from the first reactionvessel 16 to the second reaction vessel 28 at the rate of about 1640gallon per minute. The first slurry is thoroughly mixed with the secondslurry and 98% sulfuric acid which is added to the second reactionvessel at about 87 gpm. An organic sulfonic acid derivative can be addedto the second reaction vessel 28. This material is added to promote thegrowth of the hemihydrate crystals. The first slurry, the sulfuric acidand the crystal modifier are thoroughly dispersed into the second sluryin the second reaction vessel 28. The second slurry flows at the rate ofabout 1280 gallons per minute from vessel 28 into vessel 16 where it isthoroughly mixed with the first slurry.

About 45 gpm of water and volatile materials (HF, SiF₄, H₂ S, CO₂ etc.)is vaporized from the second slurry in vessel 28. Vessel 28 ismaintained at about 15 inches of mercury absolute. Approximately 400 gpmof slurry is withdrawn from the second reaction vessel and flows tovessel 48, the separator feed tank. Thus about 445 gpm of material(vaporized material and the slurry to the separator feed tank) isremoved from the system. The removed slurry is then passed to theseparation section where the solid and liquid portions of the slurry areseparated.

At these rates, the plant will produce about 350 tons per day of P₂ O₅of 35-44% P₂ O₅ phosphoric acid.

    ______________________________________                                        TOTAL LOSS IN FILTER CAKE                                                                   % of P.sub.2 O.sub.5 fed in rock                                ______________________________________                                        Citrate insoluble (CI)                                                                        0.76                                                          Citrate soluble* (CS)                                                                         4.64                                                          Water soluble (WS)                                                                            2.34                                                          Total loss      7.74                                                          Total recovery  92.26                                                         ______________________________________                                         *Lattice substitution losses determined using ammonium nitrate instead of     ammonium citrate solution.                                               

The total residence time, from entering vessel 16 to exiting vessel 48,is calculated at 7.9 hours. The volume of vessel 16 is about 120,000gallons, the volume of vessel 28 is about 40,000 gallons to normalliquid level.

Phosphate rock is present in the first and in the second slurries in thefirst and second reaction vessels respectively. The amount present isquite small and will vary considerably. The value for the "CitrateInsoluble" loss of the filter cake is a rough measure of undissolved andunreacted phosphate rock.

Examples 2 to 7

The following system as described hereinafter was set up in a pilotplant to duplicate actual plant operation in order to investigate theeffect of defoamers and crystal modifiers on the filterability, andhence the crystal size, of the calcium sulfate hemihydrate produced.

Into a first reaction vessel containing reaction slurry was addedphosphate rock, recycled phosphoric acid and recycle reaction slurryfrom the second reaction vessel. Defoamer, when used, was added in thefirst reaction vessel. The reaction slurry so formed in the firstreaction vessel was circulated to the second reaction vessel. Sulfuricacid and crystal modifiers were added to the second reaction slurry. Thesecond reaction vessel was maintained under slight vacuum so as toremove gaseous impurities and water from the slurry. The evaporation ofwater was utilized to cool the reaction slurry.

Phosphate rock is present in the first and in the second slurries in thefirst and second reaction vessels respectively. The amount present isquite small and will vary considerably. The value for the "CitrateInsoluble" loss of the filter cake is a rough measure of undissolved andunreacted phosphate rock.

Example 8

A hemihydrate system substantially as shown in FIGS. II and III andoperated substantially as described in Example 1 was discovered to bewasteful of product acid on start-up, that is, the first one or two daysof production could not be economically filtered due to fine particlesin the feed to the filter and had to be "dumped wet" with a resultingnew high loss of P₂ O₅ values.

In a modified start-up procedure, which is the invention of Gragg et al,the system was first filled with water heated by steam to 210° F. Thispreheated the vessels while testing the equipment at temperature. Thesystem was drained and filled with 185° F., 31% P₂ O₅ acid. Phosphaterock was fed to the system, while acid was recycled from the filter feedtank until the solids were builtup to about 30%, the sulfates wereadjusted to the proper level (e.g., negative in the dissolver andpositive in the crystallizer), and slurry was then sent to the filter.The plant was started up at 150 tons/day feed rate. The filter was linedout within 2 hours of startup (i.e., no wet pans after 2 hrs.). Within48 hours the design rate of 350 TPD was achieved with no apparentproblems. Product acid strength was up to 40% P₂ O₅ within one shift.

After the completion of a 44 day plant test and several modifications tothe dissolver, the plant was again started up. Essentially the samestartup procedure was used except the system was not preheated with hotwater and 35% acid @ 185° F. was used in the charging of the system. Thestartup went very smoothly with no particular problems, although thefilling of the system was slowed by evaporator problems. The firsthemihydrate cake that was produced filtered well. No filter pans weredumped wet during startup.

FIG. II illustrates a reaction system, which can be used either for ahemihydrate or gypsum process, similar to that of FIG. III but showingthe incorporation of a splitter box (to provide a head or vacuum sealand also a break to prevent siphoning) and a rock box (to trap hardpebble-like masses which apparently accumulate upon and break off fromthe sulfuric acid sparger in the crystallizer). Also shown are thecondenser between the crystallizer and the vacuun pump (which condensessteam and fluorine compounds), a scrubber to protect the vacuum pumpfrom fluorine compounds and an entrainment separator or disengager (toprevent nongaseous matter, mostly droplets containing fluorides, fromentering the vacuum pump) and the filter (which is the preferred meansof separating the calcium sulphate solids from the product phosphoricacid). The splitter box is a compartmented vessel with an entry linewhich can be moved to the top of either compartment (for slurry from thecrystallizer) at the top of one compartment and two exit lines, one atthe bottom of each compartment. One exit line is for conducting slurryto the filter feed tank, the other leads to a drain and is used forcleaning the line and the splitter box.

The present process and system can be used in practice of the inventionof Ser. No. 676,559, especially in a process for the production of highpurity phosphoric acid from phosphate rock containing metal impuritiescomprising calcium, magnesium, aluminum, ferric and ferrous iron asdescribed therein and in the applications of Ore et al, filed Dec. 29,1977.

FIG. V is a photograph of the preferred working enbodiment of theinvention where air sparge pipes can move freely in support collars.

FIG. VI shows the air sparge pipes with rigid supports.

This application is related to each of the following U.S. PatentApplications:

    ______________________________________                                        Serial Number      Filing Date                                                ______________________________________                                        703,138            July 7, 1976                                               703,139            July 7, 1976                                               703,208            July 7, 1976                                               865,556            December 29, 1977                                          865,557            December 29, 1977                                          866,963            January 4, 1978                                            866,987            January 5, 1978                                                               now abandoned                                              866,988            January 5, 1978                                            866,989            January 5, 1978                                            866,990            January 5, 1978                                                               now abandoned                                              867,556            January 6, 1978                                                               now abandoned                                              840,791            October 11, 1977                                           810,484            June 27, 1977                                              827,741            August 25, 1977                                            ______________________________________                                    

Each of these 14 patent applications is incorporated herein by thisreference as is the application of Ore, Ellis, and Moore entitled"Phosphoric Acid Process with High Circulation Rates" United States Ser.No. 909,899 now U.S. Pat. No. 4,277,448 issued July 7, 1981 and theapplication of Moore, Ore and Ellis titled "Apparatus Useful for WetProcess Phosphoric Acid Production United States Ser. No. 910,163 nowU.S. Pat. No. 4,260,584 issued Apr. 7, 1981," both filed on the same dayas the subject application.

What is claimed is:
 1. Apparatus for reacting a particulate solid and aliquid, said apparatus comprising:(a) vessel means for containing theparticulate solid and the liquid; (b) mixing means for circulating theparticulate solid and the liquid within said vessel means, said mixingmeans including a draft tube disposed in a vertical position within saidvessel means, and an agitator positioned within said draft tube, saiddraft tube having an outwardly flared lower skirt portion terminating inthe bottom portion of said vessel means and said agitator including apropeller and a drive shaft connected therewith, said drive shaft beingcoaxially mounted within said vessel means and extending into said drafttube; (c) inlet means for introducing the particulate solid and theliquid into said vessel means draft tube having an outwardly flaredlower skirt portion terminating in the bottom portion of said vesselmeans; and, (d) a source of gas and at least one gas injection meansconnected thereto for preventing appreciable settling of solids withinthe vessel means, said gas injection means extending into said outwardlyflared lower skirt portion of said draft tube.
 2. Apparatus as definedin claim 1, said inlet means having an elongated portion extendingdownwardly within said vessel means and having an external upper ventportion and an inlet portion connected to said external upper ventportion, said solid and liquid being fed into said inlet portion. 3.Apparatus as defined in claim 1, wherein said vessel means is closed andsaid inlet means has an essentially vertical conduit portion positionedwithin said vessel means, and has a vertical external upper vent portionextending above said vessel means and an inlet portion external of saidvessel means and connected at an angle to said vertical external uppervent portion of said inlet means, said solid and liquid being fed intosaid inlet portion, and said interconnected vertical external upper ventportion of said inlet means including a vent pipe, connected to saidinlet means for reducing foaming generated in said vessel means and toprevent blockage of said inlet means.
 4. Apparatus as defined in claim1, said agitator being positioned in the upper portion of said drafttube.
 5. Apparatus as defined in claim 1, wherein the inlet meansterminates in said draft tube below said agitator.
 6. Apparatus asdefined in claim 1, said vessel means having an outlet in the lower endthereof below said draft tube.
 7. Apparatus as defined in claim 6further comprising an outlet conduit means connected to the outlet,slurry recycle conduit means connected to the outlet conduit meansextending from the outlet conduit means into the vessel meansterminating in an upper portion of an annulus formed between the drafttube and the wall of the vessel means.
 8. Apparatus as defined in claim1, including an annulus between the draft tube and the vessel meansadjacent to the lower end portion of said draft tube and an overflowpipe extending from the upper end portion of said vessel means, and arecycle slurry pipe extending into said vessel means and terminating ata zone in said annulus.
 9. Apparatus as defined in claim 1, saidagitator being positioned in the lower portion of said draft tube. 10.Apparatus as defined in claim 1, said vessel means having an inner walladjacent the bottom thereof, said inner wall being outwardly curved fromthe lower end of said draft tube.
 11. Apparatus as defined in claim 10,including an annulus between the draft tube and the vessel meansadjacent to the lower end portion of said draft tube and an overflowpipe extending from the upper end portion of said vessel means, conduitmeans extending into said vessel means and terminating at a zone in saidannulus for recycling slurry into said vessel means and a second inletmeans disposed in said vessel means for introducing a defoaming agentthereto.
 12. Apparatus as defined in claim 1 wherein said gas injectionmeans comprises at least one means for introducing gas outside the drafttube and one means for introducing gas under said draft tube and intothe lower interior of said draft tube.
 13. Apparatus as defined in claim1 wherein the apparatus includes a source of phosphate rock and a sourceof a strong inorganic acid.
 14. An apparatus for the production ofphosphoric acid by the reaction of phosphate rock, sulfuric acid andphosphoric acid, said apparatus comprising at least a first reactionvessel and a second reaction vessel interconnected with piping for thesimultaneous transfer of slurry between the first and second reactionvessels, each reaction vessel having a draft tube extending verticallytherein and having an outwardly flared lower skirt portion terminatingin a bottom portion of each reaction vessel whereby an annulus iscreated between the reaction vessel and the draft tube, and a propelleragitator positioned within each draft tube and mounted on a shaftextending vertically and axially of each reaction vessel, the firstreaction vessel having an inlet means connected to a source of aphosphate rock slurry for the introduction of phosphate rock slurry tothe first reaction vessel and the second reaction vessel having an inletmeans connected to a source of sulfuric acid for the introduction ofsulfuric acid to the second reaction vessel, the improvement in theapparatus comprising an inlet conduit means on the first reaction vesselextending into the draft tube in the first reaction vessel, the inletconduit means having an essentially vertical portion within the firstreaction vessel, an external upper vent portion vertically extendingabove the first reaction vessel connected at an angle to the verticallyextending upper vent portion for introducing a feed slurry of phosphaterock to the first reaction vessel; at least one gas introducing meansfor introducing a gas to the bottom portion of at least one of the firstand second reaction vessels and below the outwardly flared lower skirtportion of the draft tube in said reaction vessel, said gas introducingmeans being operative for providing a sufficient fluid velocity forsuspending solids present in each reaction vessel and preventingappreciable settling of solids; and in said second reaction vessel, asecond inlet conduit means connected to a source of sulfuric acid forthe introduction of sulfuric acid in the second reaction vessel.
 15. Theimproved apparatus as defined in claim 14 further comprising meansprovided on the first and second reaction vessels for recycling slurryexternally of each reaction vessel back to the reaction vessel.
 16. Theimproved apparatus as defined in claim 15 wherein each reaction vesselhas an inlet means for introducing a defoaming agent into the reactionvessel.
 17. The improved apparatus as defined in claim 16 wherein theinlet means for introducing a defoaming agent into the first reactionvessel is disposed on the inlet portion of the inlet conduit means. 18.The improved apparatus as defined in claim 14 wherein the means forintroducing a gas to the lower interior portion of each reaction vesselcomprises at least one additional gas introducing means positioned forintroducing gas in the annulus created between the reaction vessel andthe draft tube.